A conventional surface acoustic wave device is explained below by referring to the drawing. FIG. 6 is a top view of the conventional surface acoustic wave device.
In FIG. 6, surface acoustic wave device 1 includes piezoelectric element 2 made of, for example, lithium niobate, lithium tantalate, or the like, and inter-digital transducer (hereinafter called IDT) electrode 3 made of copper or other metal formed on this piezoelectric element 2. Further, on piezoelectric element 2, other components are mounted, that is, grating reflectors 7 provided at both ends of IDT electrode 3, pad electrode 40 made of aluminum or other metal electrically connected to IDT electrode 3, and light permeable dielectric layer 5 made of silicon oxide or the like formed on piezoelectric element 2 for covering IDT electrode 3. This light permeable dielectric layer 5 has opening 6 to be exposed from light permeable dielectric layer 5 in part or in whole of the top of pad electrode 40 so as to be conductive between the outside of the device and pad electrode 40.
In a manufacturing process of such conventional surface acoustic wave device 1, after forming a film of light permeable dielectric layer 5, by comparing the polarized state of the incident light and the polarized state of the reflected light reflected from piezoelectric element 2 by using an ellipsometer or the like, the film thickness of light permeable dielectric layer 5 is measured.
However, in such measurement of the film thickness, the light reflectivity of piezoelectric element 2 in a visible light wavelength region is not sufficiently high, or the measured value is very sensitive to roughness of the substrate surface, and it was difficult to measure the film thickness accurately. As a result, surface acoustic wave devices out of the frequency standard may be produced and the yield is poor, and the productivity is lowered. These problems are caused because of large change in frequency characteristic of the surface acoustic wave device depending on the film thickness of light permeable dielectric layer 5.
The present invention is hence devised to enhance the productivity of surface acoustic wave devices by improving the accuracy of film thickness measurement of the light permeable dielectric layer.